1. Field of the Invention
The present invention relates to an electric double layer capacitor, and more particularly, it relates to an improvement in structure of an electric double layer capacitor having a plurality of electric double layer capacitor elements stacked and held in a metal case.
2. Description of the Prior Art
FIG. 2 shows an example of a conventional electric double layer capacitor, which comprises a plurality of electric double layer capacitor elements 2, 3, . . . , 4 received in a cylindrical metal case 1. An opening 1a is defined in one end of the cylindrical metal case 1 hereinafter referred to for convenience as the upper end, and a terminal board 5 is provided in the upper end of the metal case 1 to close the opening 1a. The terminal board 5 comprises a first conductive plate 6 which is in contact with the upper surface of the electric double layer capacitor element 2, an insulating plate 7 having in a lower cavity thereof said first conductive plate 6, and a second conductive plate 8 provided on the upper surface of the insulating plate 7. Outwardly extending lead wires 6a and 8a extend from the conductive plates 6 and 8 respectively.
In the conventional electric double layer capacitor as shown in FIG. 2, the peripheral edge of the opening 1a in the metal case 1 is inwardly crimped to hold the terminal board 5 and the stacked electric double layer capacitor elements 2 . . . 4, which are in close contact with each other, in the metal case 1.
Although not clearly shown in FIG. 2, the second conductive plate 8 is electrically connected to the metal case 1 by solder, for example.
As seen in FIG. 2, the outer surface of the terminal board 5, i.e., the upper surface of the second conductive plate 8, is merely pressed by a crimped portion 1b of the metal case 1. In order to mount the electric double layer capacitor of FIG. 2 on a printed circuit board or the like, the lead wires 6a and 8a are soldered to portions to which the lead wires are to be electrically connected, and the electric double layer capacitor is washed by a solvent after such soldering.
In the structure as shown in FIG. 2, a clearance between the second conductive plate 8 and the crimped portion 1b of the metal case 1 is not necessarily reliably sealed but merely the edge of the metal case 1 is crimped, whereby the solvent employed for washing may enter the interior of the metal case 1 through the clearance between the forward end of the crimped portion 1b and the second conductive plate 8 to cause shorting between the electric double layer capacitor elements 2 . . . 4.